The present invention relates to a semiconductor device including a semiconductor chip and leads connected with the semiconductor chip. In particular, it relates to a semiconductor device mounting a transistor dealing with a high frequency signal having an improved high frequency signal characteristic.
As a known structure of a general semiconductor device, a semiconductor chip is connected with a lead frame through bonding wires, the resultant is encapsulated with a resin, and a portion of the lead frame at the outside of the encapsulating resin is cut off.
FIG. 35 is a perspective view for showing the structure of a semiconductor device disclosed in Japanese Laid-Open Patent Publication No. 7-202111. As is shown in FIG. 35, a semiconductor chip 1002 is mounted on a die pad 1001 of a lead frame. A large number of outer leads 1003 extend from an outer frame (not shown) of the lead frame, and inner leads 1004 continuous with the outer leads 1003 extend toward the semiconductor chip 1002. Each of the tips of the inner leads 1004 is connected with each electrode pad 1005 on the semiconductor chip 1002 through a bonding wire 1006. The semiconductor chip 1002, the die pad 1001 and the inner leads 1004 placed in this state are encapsulated with an encapsulating resin 1010. However, the outer leads 1003 protrude beyond the encapsulating resin 1010 because the flow of the encapsulating resin 1010 is held back by dam bars 1007 connecting the outer leads 1003. Then, a part of the outer leads 1003 including the dam bars 1007 is cut off, so that the outer leads 1003 protruding beyond the encapsulating resin 1010 can be separated from one another. By using such a structure of the semiconductor device, signals can be sent and received between external equipment and elements such as a transistor included in the semiconductor chip 1002 through the outer leads 1003.
FIG. 36 is a plan view for showing the structure of a general lead frame disclosed in Japanese Laid-Open Patent Publication No. 6-151681. As is shown in FIG. 36, a die pad 1002 is supported by tie-bars 1008 at four corners thereof, and outer leads 1004 are respectively connected with an outer frame 1009.
As a result of improved integration of a semiconductor device, the dimension of a semiconductor chip is recently decreased, whereas the number of elements disposed in the semiconductor chip is increased. Also, a semiconductor device dealing with a high frequency signal is more frequently used.
Therefore, the aforementioned conventional semiconductor device have the following problems:
As the number of elements is increased in accordance with the improved integration, the number of electrode pads on a semiconductor chip is increased. Therefore, the number of inner leads and outer leads is naturally increased, and hence, the pitch between adjacent inner leads 1004 is unavoidably decreased. As a result, the inner leads 1004 can be made to come in contact with one another due to the flow of the encapsulating resin. In particular, a semiconductor device dealing with a high frequency signal has the following problems:
In general, voltage decrease e caused by a current i in an R-L-C serial circuit is represented by the following formula (1): EQU e=R.multidot.i+L.multidot.di/dt+(1/C).intg.idt (1)
wherein R indicates a resistance, L indicates an inductance, and C indicates a capacitance. Generally, a system in a package is a distributed constant circuit. Also, the inductance L includes self-inductance and mutual inductance.
As is understood from the formula (1), when the current i vigorously varies with time, a value of L.multidot.di/dt is increased, and a voltage corresponding to the value of L.multidot.di/dt is generated. Accordingly, when two inner leads come close to each other, a voltage is decreased in one inner lead by mutual inductance in proportion to the variation with time of a current flowing through the other inner lead. In other words, a noise due to crosstalk and switching can become serious. In particular, in a semiconductor device, such as a multi-bit DRAM having a large capacity, where a large number of transistors therein are simultaneously and rapidly operated and a large number of signal terminals therein are simultaneously turned on/off, it is very difficult to decrease such noise due to crosstalk and switching by using the conventional lead frame.
Furthermore, in a semiconductor device dealing with a high frequency signal, a voltage of a lead connected with a power supply or ground is varied owing to a self-inductance of another lead connected with another signal as well as a mutual inductance with the power supply or the ground lead. As a result, a potential difference can be caused between the ground in the semiconductor chip and the ground of external equipment or between a supply voltage in the semiconductor chip and a supply voltage of external equipment. Accordingly, in a semiconductor device such as a multi-bit DRAM having a large capacity where a large number of transistors therein are simultaneously and rapidly operated and a large number of signals are simultaneously transferred through respective lines therein, the supply potential and the ground potential can be varied with time. Thus, a signal waveform can be degraded and malfunction of the elements can be caused.
Moreover, in a semiconductor device dealing with a high frequency signal, it is actually impossible to match impedance with a lead frame used.
Additionally, as another problem, the conventional semiconductor device is required to mount a path capacitor having a large capacitance between a power line and a ground line in order to remove power noise. This has obstructed realization of compactness of the semiconductor device.